1. Field of the Art
The present invention relates generally to an electromagnetic powder clutch, and more particularly to improvements in an electromagnetic powder clutch having two powder gaps accommodating masses of magnetic powder.
2. Related Art Statement
As a clutch for automotive vehicles and other applications, an electromagnetic powder clutch is known.
Such an electromagnetic powder clutch is classified into a single-gap type and a plural-gap type. The single-gap type has a single powder gap in which a mass of magnetic powder is accommodated. The plural-gap type has a plurality of powder gaps, generally two powder gaps.
While an electromagnetic powder clutch of the single-gap type is advantageous for its simple construction and comparatively high operating stability, its torque transmission capacity is smaller than the plural-gap type. Consequently, the powder clutch of the single-gap type must be comparatively large-sized to provide the same torque transmission capacity as the powder clutch of the plural-gap type. This is a disadvantage of the single-gap type.
On the other hand, an electromagnetic clutch of the plural-gap type has a relatively large capacity with a relatively small size. For this reason, the plural-gap type is used when space for installation is limited.
An electromagnetic powder clutch of the plural-gap type is either of a non-separate gap type or of a separate gap type. In a powder clutch having two powder gaps, for example, the two powder gaps are continuous or separate. More specifically, the non-separate gap type has a mass of magnetic powder which may be moved from one powder gap to another, while the separate gap type has two masses of magnetic powder which are accommodated in two mutually independent powder gaps. In the separate gap type, the powder masses are not permitted to meet each other.
However, the non-separate or continuous gap type is unstable in torque transmission capability, because the powder mass may be moved between the two powder gaps. This is a problem with a powder clutch of the non-separate gap type.
When the installation space is limited and high torque transmission capability is desired, it has been a practice to use an electromagnetic powder cluch of the plural separate gap type. This type of powder clutch is satisfactory in its torque transmission stability, because each of the powder masses is inhibited from moving from one powder gap to another, i.e., the powder mass is enclosed in the corresponding independent powder gap, like the powder mass in a single powder gap.
However, a conventional powder clutch of two separate gap type tends to have a larger dimension in the axial direction of the clutch than the clutch of the plural non-separate gap type. Described in more detail, a commonly used powder clutch of the two separate gap type has two annular powder gaps on both inner and outer sides of an annular member. The two powder gaps are spaced from each other in the radial direction of the clutch. These two annular powder gaps are closed at their opposite axial ends by four respective labyrinth units to prevent the powder masses from being discharged from the corresponding powder gaps. The four labyrinth units are disposed at each end of the two powder gaps, in spaced-apart relation in the axial direction of the clutch. On the other hand, a powder clutch of two non-separate or continuous gap type uses only two labyrinth units, at one axial end of each powder gap, which two labyrinth units are spaced from each other in the axial direction. Accordingly, the axial lenth of the two separate gap type is larger than the two non-separate gap type, by a distance corresponding to the two spaced-apart labyrith units used at each end of the two powder gaps.
In view of the above, there has been a need of minimizing the overall axial length of a powder clutch for the plural separate gap type by means of reducing the length necessary for provision of a labyrinth device to enclose powder masses in respective powder gaps.
In a powder clutch of two gap type having a radially outer annular gap and a radially inner annular gap, it is needless to say that the diameter of the outer annular gap is larger than that of the inner annular gap. Hence, the area of an outer friction surface of the radially outer annular gap is larger than that of the radially inner annular gap if the two annular gaps have the same axial length. Consequently, a maximum transmission torque is obtained when a maximum magnetic flux density is reached on the outer friction surface of the radially inner annular gap. Namely, the maximum transmission torque of the clutch is determined by the area of the outer friction surface of the inner annular gap. Further, since the area of the outer friction surface of the inner annular gap is smaller than that of the outer annular gap, the magnetic flux density is higher on the friction surface of the inner annular gap than on the friction surface of the outer annular gap. Therefore, the transmission torque and the amount of heat generated per unit area are larger on the friction surface of the inner annular gap than on the friction surface of the outer annular gap. This fact, combined with relatively poor heat dissipation from the radially inner portion of the clutch, will lead to reduced durability of the electromagnetic power clutch.